This invention relates to methods and apparatus for reducing NOx in engine exhaust and more particularly to method and apparatus for reducing NOx in engine exhaust by injecting a reactant to react with such engine exhaust.
As is known in the art, lean burn engines (e.g., diesel and DISI engines) provide great fuel efficiency compared to stoichiometric spark ignited engines at the expense of more complicated exhaust after-treatment. More particularly, one such after-treatment is the reduction of engine exhaust NOx. Lean NOx catalysts (ALNCs) are typically utilized to reduce tail pipe NOx emissions.
In a typical ALNC configuration, a reductant or reactant, e.g., urea or hydrocarbon, is introduced into the engine exhaust stream. In many cases, a catalyst is used to facilitate the reaction This NOx reduction arrangement is in the case of a hydrocarbon (HC) reactant essentially an open-loop arrangement because a measurement of the effectiveness of the NOx reduction is not used to adjust the amount of reactant being introduced, or injected into the engine exhaust. This open-loop arrangement includes a look-up table which stores the relationship between the desired amount of hydrocarbon injection in accordance with engine speed, engine load, EGR level, start of fuel injection, catalyst temperature and space velocity, inter alia. Typical injection strategies compute the HC quantity q1 to be injected as the product of a first function f1 (where f1 is a function itself of space velocity (SV), engine speed (RPM) and fuel quantity ) and a second function, f2, which is a function of catalyst temperature, Tcat.
More particularly, q1=f1(SV, RPM, fuel)*f2(T_cat). Thus, f1 and f2 are determined a priori to thereby compute q1. The signal representative of q1 is used as the control signal for an HC injector.
The inventors have recognized that there is no way to guarantee that the desired hydrocarbon quantity injected is actually injected since the actual hydrocarbon concentration in the engine exhaust cannot be measured. Injector uncertainty (e.g., injector plugging) may cause the actual injected quantity to be more or less than the desired quantity. We have recognized that since advanced diesel treatment packages include temperature sensors before and after the ALNC, a calculation of the exotherm across the ALNC can be performed to thereby provide an indication of whether the desired hydrocarbon quantity injected is actually injected into the engine exhaust. Also, feedgas HC may change due to engine ageing. This method allows the HC injector to compensate for such changes.
In accordance with the present invention, a method and system are provided for determining whether a desired amount of a reactant is actually injected into an engine exhaust to react with NOx in such engine exhaust. The reaction is facilitated with a catalyst downstream of the reactant injection. The method and system measure a quantity of heat generated from the reaction across the catalyst during the reaction. The measure heat quantity is compared with a quantity of heat expected to be generated across the catalyst during the reaction.
In one embodiment the quantity of heat is proportional to the temperature difference produced across the catalyst. In the case of hydrocarbon injection, the quantity of heat is the amount of heat generated by an exothermic reaction between the hydrocarbon and the O2 in the engine exhaust. This generated heat is referred to as an exotherm.
With such an arrangement, use of the pair of temperature sensors disposed across the catalyst enables a processor to detect whether a desired hydrocarbon quantity injected is actually injected into the engine exhaust.
In accordance with one embodiment, a method and system are provided for determining whether a hydrocarbon injector used to inject hydrocarbons into an engine exhaust to react with NOx in such engine exhaust is degraded. The method and system include measuring an exotherm produced across the catalyst during the reaction and comparing the measured exotherm with an expected exotherm across the catalyst during the reaction.
In one embodiment, a method and system are provided for determining whether a hydrocarbon injector used to inject hydrocarbons into an engine exhaust to react with NOx in such engine exhaust is degraded. The method and system include measuring an exotherm across the catalyst during the reaction; and determining an exotherm difference, such exotherm difference being the difference between the measured exotherm and an exotherm expected to be produced across the catalyst during the reaction.
In one embodiment, the method and system include integrating the exotherm difference over time; and comparing the integration result with predetermined level to detect whether the injector is degraded.
In accordance with another feature of the invention, a method and system are provided for controlling injection of a reactant into a substance to be reduced by such reactant. The reaction is facilitated by a catalyst. The method and system measure a quantity of heat generated from the reaction across the catalyst during the reaction. The measured heat quantity is compared with a quantity of heat expected to be generated across the catalyst during the reaction. A heat quantity difference is determined. The heat quantity difference is the difference between the measured heat quantity and the expected heat quantity difference; and adjusting the amount of reactant injection in accordance with the determined heat quantity difference.
In one embodiment, a method and system are provided for controlling injection of a reactant into an engine exhaust. The exhaust includes a substance to be reacted with the reactant to reduce such substance. The method and system included: measuring an exotherm produced during the reaction; determining an exotherm difference, such exotherm difference being a difference between the measured exotherm and an expected exotherm; and changing an amount of reactant injection by integral control if the determined exotherm difference exceeds a threshold, otherwise maintaining the amount of reactant injection level.
In one embodiment the reactant is a hydrocarbon and the quantity of heat is an exotherm (i.e., the amount of heat associated with an exothermic reaction between the hydrocarbon and substance.)
In one embodiment the substance is NOx in the engine exhaust.
In accordance with another feature of the invention, a system is provided for controlling injection of a hydrocarbon into an engine exhaust. The exhaust includes NOx to be reacted with the hydrocarbon to reduce such NOx. The system includes a pair of temperature sensors, one of such sensors being disposed upstream of a catalyst and the other one of the temperature sensors being disposed downstream of the catalyst. A hydrocarbon injector is provided for injecting, in response to a control signal, the hydrocarbon into the engine exhaust upstream of the catalyst. A look up table is included for determining a nominal portion of the control signal.
The nominal portion of the control signal represents a nominal amount of the hydrocarbon to be injected into the engine exhaust. The nominal amount is a function of a plurality of operating parameters including engine operating conditions and catalyst temperature. A variable control signal generator is provided for producing a time variable portion of the control signal. The time variable portion of the control signal is a function of a difference in temperatures measured by the pair of temperature sensors.